Conventionally, there have been proposed a variety of data storage systems each incorporating a number of HDDs as data storage elements rather than tapes, for example, from Sun Microsystems Inc., 901 San Antonio Road, Palo Alto, Ga. 94˜03 USA, under the tradename Sun StorEdge 9900 series. According to the HDD data storage system, each HDD fixed and arranged in a shelf equipped in a cabinet is always powered so that data can immediately be written in and retrieved from the HDD in response to data input and output instruction, respectively.
Disadvantageously, this consumes a great deal of electric power. Also, an electric fan must be prepared for the dissipation of heat generated from HDDs and thereby to maintain a constant temperature within an interior of the cabinet. Further, the necessity of supplying electric power to all of the HDDs will invite an excessive enlargement of UPS (Uninterruptible Power Supply)—Furthermore, a service life of HDD is shortened because a power is constantly supplied thereto regardless of data I/O usage, resulting in frequent replacement of storage elements. Moreover, a requirement for HDD to be used as a storage element to a host computer needs an associated interface channel and uninterrupted connection, limiting the total number of storage elements and overall capacity any single host can connect with.